Linear Compressor and Drive Unit Therefor

ABSTRACT

A drive unit for a linear compressor having a frame and a body configured for reciprocating movement connected to the frame by at least one diaphragm spring and means for guiding the body to allow linear reciprocating movement with respect to the frame, the drive unit including a coil spring for action on the reciprocating body and the frame, the coil spring being configured for extension and compression in a direction of movement.

The present invention relates to a linear compressor, in particular foruse in compressing refrigerant in a refrigerating device, and a driveunit for driving an oscillating piston movement for such a linearcompressor.

U.S. Pat. No. 6,596,032B2 discloses a linear compressor whose drive unitcomprises a frame and an oscillating body mounted in the frame via adiaphragm spring. The oscillating body comprises a permanent magnet, apiston rod rigidly connected to the permanent magnet, and, connected byan articulated joint to the piston rod, a piston that can move with areciprocating motion in a cylinder. The movement of the piston is drivenby an electromagnet disposed all around the cylinder that interacts withthe permanent magnet. A disc-shaped diaphragm spring is screwed onto thepiston rod in the center, and the outer edge of the diaphragm spring isconnected to a yoke that surrounds the cylinder, the electromagnet andthe permanent magnet.

Compared with many other types of spring, the diaphragm spring has theadvantage that it can only be deformed with difficulty at right anglesto the oscillation direction. Hence the oscillating body can only movewith one degree of freedom, unlike an oscillating body suspended from acoil spring, for example, which always has three degrees of freedom oftranslational motion, and requires a guide if the motion is to berestricted to a single degree of freedom. Such a guide is not requiredfor an oscillating body supported on a diaphragm spring. Hence themovement of such an oscillating body can be converted with low frictionlosses into the movement of a piston in a compressor, which isnecessarily guided along a strictly linear path.

The oscillating body and the diaphragm spring form an oscillatory systemwhose resonant frequency is determined by the mass of the oscillatingbody and of the diaphragm spring, and the stiffness of the diaphragmspring. The diaphragm spring permits only small oscillation amplitudesbecause each deflection of the oscillating body is associated with anextension of the diaphragm spring. The small oscillation amplitude meansit is difficult to make the dead volume of the cylinder reliably small.The larger the dead volume, however, the lower the efficiency of thecompressor. The short travel also compels the cylinder to be designedwith a large diameter relative to the length in order to achieve a givencapacity. It is costly to seal the correspondingly large pistoncircumference.

Another option for increasing the capacity is to make the diaphragmspring very stiff in order to increase the resonant frequency thereby.The stiffer the diaphragm spring, however, also means that there is agreater risk of the spring suffering fatigue for a given oscillationamplitude. This means that, in order to avoid fatigue, the amplitudemust be made smaller the stiffer the spring, so that again asatisfactory increase in capacity cannot be achieved in this way.

The object of the present invention is to create a drive unit for alinear compressor having a frame and an oscillating body mounted in theframe via a diaphragm spring, in which the diaphragm spring allows largetravel of the oscillating body without the risk of fatigue, so that ahigh capacity can be achieved for a small piston diameter.

The object is achieved in that a coil spring, in addition to thediaphragm spring, is attached to the oscillating body and the frame andcan be extended and compressed in the direction of movement. It isthereby possible to split the functions of guiding the oscillating bodyand of temporary storage of its kinetic energy. The coil spring is onlyslightly suited to constraining the oscillating body along an exactlydefined straight line, but it is not difficult to dimension it tosustain both a desired amplitude of movement and a desired frequency ofmovement of the oscillating body without the risk of material fatigue.The diaphragm spring must have only a small material thickness in orderto achieve a desired large oscillation amplitude. Such a diaphragmspring would only permit a low resonant frequency of the oscillatingbody were it the sole mechanism having to perform the function oftemporary energy storage. By connecting the two types of springs inparallel, however, all three requirements can be achievedsimultaneously, namely the requirements for strict guidance of theoscillating body, a large amplitude and a high oscillating frequency.

Ideally, the springs should only exert forces but no turning moments onthe oscillating body. For this purpose, the coil spring is preferablydisposed around an imaginary straight line along which the center ofgravity of the oscillating body can perform a reciprocating motion. Thestraight line preferably coincides with a longitudinal axis of the coilspring.

In order to prevent the diaphragm spring exerting a turning moment, orin order to minimize such a turning moment, the diaphragm springpreferably has an axis of symmetry that coincides with the straightline, or a plane of symmetry on which the straight line lies.

In order to transfer the force from the coil spring into the oscillatingbody without any turning moment, one end of the coil spring preferablyacts on the circumference of a spring plate to whose center theoscillating body is attached.

In order to make the diaphragm spring slightly deformable in thedirection of movement, it preferably has a plurality of bent arms, oneend of each arm being fixed to the frame and another end to theoscillating body.

In order to improve the accuracy with which the oscillating body isguided along the straight line, at least two diaphragm springs arepreferably provided, which act on areas of the oscillating body that areset apart in the direction of the oscillating movement.

The subject of the invention is also a linear compressor having aworking chamber, a piston performing a reciprocating motion in theworking chamber in order to compress a working fluid, and a drive unitas defined above, which is coupled to the piston to drive thereciprocating motion. In order to make such a linear compressor compact,it can be advantageous for the working chamber to be surrounded at leastpartially by the coil spring.

Further features and advantages of the invention follow from thedescription below of exemplary embodiments with reference to theenclosed figures, in which:

FIG. 1 shows a perspective view of a linear compressor according to theinvention;

FIG. 2 shows one of the two diaphragm springs of the linear compressorof FIG. 1;

FIG. 3 shows a schematic section through part of the linear compressoralong an imaginary straight line G;

FIG. 4 shows an alternative embodiment of the diaphragm spring of thelinear compressor; and

FIG. 5 shows a further simplified embodiment of the diaphragm spring.

A frame 1 of the linear compressor comprises a base plate 2 from whichextend protrusions 3, 4, 5 in the form of plates or ribs. Two diaphragmsprings 6 of the type shown in FIG. 2 are screwed onto the narrow sidesof the two facing protrusions 3. The diaphragm springs 6 each compriseedge sections 7, which rest against the end faces of the protrusions 3and from whose ends extend Z-shaped or S-shaped spring arms 8. The endsof the spring arms 8 remote from the edge sections 7 meet each other ina center section 9 of the diaphragm spring 6 in which three holes 10, 11are formed. An oscillating body 12 is fixed between the two diaphragmsprings 6 using screws or rivets (not shown), which extend through theupper and lower holes 10 of the diaphragm springs 6. The hole 11 forms apassage for a piston rod 13, which extends between the oscillating body12 and a compressor assembly 14 carried by the protrusion 5.

Two electromagnets 15 are arranged on either side of the permanentlymagnetic oscillating body 12 in a hollow space bounded by theprotrusions 3 and the diaphragm springs 6, with current being able toflow through said electromagnets in order to generate between themopposite magnetic fields to each other, which deflect the oscillatingbody 12 out of its equilibrium position shown in FIG. 1 along a straightline G running through the center of gravity of the oscillating body 12in the one or the other direction.

The straight line G runs axially through the piston rod 13 and thecompressor assembly 14, and simultaneously forms the axis of symmetry oftwo spring plates 16, which are pressed by coil springs 17 against theouter faces of the two diaphragm springs 6. FIG. 3 shows a longitudinalsection through part of the linear compressor along this straight lineG. The spring plates 16 each have a ridge running around the edge oftheir concave side facing away from the diaphragm springs 6, which fixesin a radial direction a last turn of the coil spring 17 resting againstthe spring plates 16. The opposite ends of the coil springs 17 are eachfixed by protrusions extending inside the springs. One is a flatprotrusion 18 on the plate 4 of the frame 1; the other protrusion 19 ispart of the compressor housing 14.

The coil springs 17 are each stretched between the spring plates 16 andthe protrusions 18 or 19 that support them in such a way that at noreversal point of the movement of the oscillating body 12 is one of thecoil springs 17 not under tension. The coil springs 17 hence constantlypress the spring plates 16 against the diaphragm springs 6, even whenthe compressor is operating and the oscillating body 12 is oscillating.Hence there is no need for the spring plates 16 to be fixed to thediaphragm springs 6 that they touch in order to maintain constantcontact between them. Since the force of the springs 17 acts on each ofthe spring plates 16 in a fairly evenly distributed manner over theentire area of the spring plates 16, a low turning moment does resultthat could cause tilting of the axes of the spring plates with respectto the straight line G. Even if such a turning moment were to occur,however, since there is no physically linked connection between thespring plates 16 and the diaphragm springs 6, this moment could not betransferred to the latter. Owing to the spring plates 16 being taperedtowards the diaphragm springs 6, they transfer the force of the coilsprings 17 into the diaphragm springs 6 very closely along the line G,so that even a turning moment acting on the diaphragm springs 6resulting from an uneven force distribution remains small. Hence thediaphragm springs 6 and the oscillating body 12 supported by them issubjected by the coil springs 17 to forces aligned substantially onlyexactly in the direction of the straight line G but to negligibleturning moments that could excite movement of the center of gravity ofthe oscillating body 12 outside the line G.

The high degree of symmetry of the two diaphragm springs 6 alsocontributes to their guiding the oscillating body 12 exactly along aline.

The section in FIG. 3 also shows the internal design of the compressorassembly 14. A piston 21 held by the piston rod 13 can performreciprocating motion in an internal chamber 20 of the compressorassembly 14 in order to suck refrigerant into the chamber 20 via suctionpipe 22, and output the compressed refrigerant again at a pressure pipe23. An annular space 24 extending in a cup shape around the chamber 20communicates with the pressure pipe 23. The edges of the piston 21 brushalong the dividing wall 25 between the chamber 20 and the annular space24, in which are formed a multiplicity of narrow passages 26 throughwhich some of the compressed refrigerant can flow out of the annularspace 24 back into the chamber 20. The returning refrigerant forms a gascushion between the dividing wall 25 and the edges of the piston 21,this cushion preventing direct frictional contact between piston 21 anddividing wall 25 and hence keeping down wear of the compressor assembly14. By virtue of the oscillating body 12 being guided exactly in astraight line, which is achieved by suspending by diaphragm springs andcoil springs 6, 17, a low gas flow rate in the passages 26 is sufficientto create a gas cushion effective in protecting against friction.

In order to compensate for slight inaccuracies in the mutual alignmentof the drive unit and the compressor assembly, which could otherwisealso result in the piston 21 rubbing against the wall 25, twoelastically deflectable weak points 27 are formed in the piston rod 13.A slight deflection of these weak points 27 makes it possible tocompensate for a small offset between the straight line G along whichthe center of gravity of the oscillating body 12 moves and the centrallongitudinal axis of the chamber 20 or even to compensate for a slightnon-parallelism between the two.

Simplified embodiments of the diaphragm spring are shown in FIGS. 4 and5. The spring 6′ of FIG. 4 essentially corresponds to half a diaphragmspring from FIG. 3, having just two arms bent into an S-shape or aZ-shape, which extend from an edge section 7 to the center section 9. Inthe spring 6″ of FIG. 5, the bent arms are replaced by a straight arm8″. Although strictly speaking its free end does not move exactly alonga straight line but along an arc, this deviation is negligible if theamplitude of the oscillating body is limited so that the sidewayscomponent of the movement of the oscillating body is smaller than thelateral play of the piston.

1-11. (canceled)
 12. A drive unit for a linear compressor having a frameand a body configured for reciprocating movement connected to the frameby at least one diaphragm spring and means for guiding the body to allowlinear reciprocating movement with respect to the frame, the drive unitcomprising a coil spring for action on the reciprocating body and theframe, the coil spring being configured for extension and compression ina direction of movement.
 13. The drive unit according to claim 12wherein the coil spring is configured to extend around a straight linealong which the center of gravity of the reciprocating body can move inreciprocating manner.
 14. The drive unit according to claim 13 whereinthe straight line is coincident with a longitudinal axis of the coilspring.
 15. The drive unit according to claim 13 wherein the straightline is at least a part of an axis of symmetry of the diaphragm spring.16. The drive unit according to claim 12 and further comprising a springplate wherein one end of the coil spring acts on the circumferencethereof, and wherein a center of the spring plate presses against thereciprocating body.
 17. The drive unit according to claim 12 wherein thediaphragm spring comprises a plurality of bent arms wherein one end ofeach arm is fixed to the frame and another end of each arm is fixed tothe reciprocating body.
 18. The drive unit according to claim 17 whereineach arm includes two sections bent in different directions.
 19. Thedrive unit according to claim 12 and further comprising at least asecond diaphragm spring, wherein the first and the second diaphragmsprings act on areas of the reciprocating body that are set apart in thedirection of the reciprocating movement.
 20. A linear compressor havinga working chamber, a piston configured for reciprocating movement in theworking chamber in order to compress a working fluid, and a drive unithaving a frame and a reciprocating body connected to the frame by atleast one diaphragm spring; and means for guiding the reciprocating bodyin a linear reciprocating movement with respect to the frame, the driveunit comprising a coil spring for action on the reciprocating body andthe frame, the coil spring being configured for extension andcompression in the direction of movement, wherein the drive unit iscoupled to the piston to drive the reciprocating movement.
 21. Thelinear compressor according to claim 20 and further comprising a pistonrod extending between the piston and the reciprocating body along astraight line.
 22. The linear compressor according to claim 20 whereinthe working chamber is at least partially surrounded by the coil spring.